Heavy oil is particularly difficult to upgrade in refinery operations. Metals contained in the oil tend to rapidly deactivate catalysts with which they come in contact during the upgrading process. Heavy oils often contain high concentrations of sulfur and nitrogen, which are difficult to remove to the extent necessary for further processing of the upgraded products from heavy oil processing. The aromatic character of many heavy oils tends to contribute to instability of the upgraded products. While heating the heavy oils, even in the presence of high pressure hydrogen, components of the heavy oil thermally crack to yield free radicals, which quickly combine to make sediment and coke precursors unless they are quickly suppressed by active catalysis. Furthermore, during catalysis, high molecular weight coke precursors deposit on catalysts and quickly reduce catalytic activity.
Rapid deactivation of catalysts used in heavy oil processing service often requires frequent replacement of catalysts in the heavy oil processing systems. Several systems have been proposed for replacing a portion of the catalyst at regular intervals during the heavy oil processing, without requiring that the system be shut down for catalyst replacement. In an ebullated bed heavy oil processing system, the catalyst is maintained in a fluidized state within the reaction zone. At periodic intervals, a portion of the fluidized bed of catalyst, along with a small portion of fluidizing liquid, is removed from the system. A comparable amount of catalyst is added to the system, to maintain a constant quantity of catalyst in the system at any one time.
An ebullated bed processing system for use in heavy oil processing has a fairly low catalyst/oil ratio within the reaction zone. It is desirable to increase the catalyst/oil ratio to improve the overall effectiveness of the system, without requiring significant modifications to the system. U.S. Pat. Nos. 7,815,870; 7,449,103; 8,024,232; 7,618,530, and US Patent Publication Nos. 2011/0226667 and 2009/0310435 disclose ebullating bed hydroprocessing systems wherein the catalyst system comprises both a porous supported catalyst and a “colloidal” catalyst for the upgrade of heavy oil feedstock. The single-metallic colloidal catalyst employed is synthesized in-situ upon mixing with the heavy oil feedstock under sufficient conditions for sulfidation to occur; thus tight control of the catalyst properties is difficult. A process employing an in-situ synthesized catalyst requires carefully controlled steps for the dilution of the catalyst precursor and mixing with a heavy oil feedstock for sulfidation to take place.
There is still a need for an improved reaction feed system with improved properties and performance for heavy oil conversion processes.